This invention relates to an optical disc recording device for radiating a laser beam on an optical disc for recording data thereon.
There has so far been known an optical disc device in which a laser light beam is radiated on a disc-shaped recording medium for sequentially forming pits for recording the information thereon. Such optical disc device may be exemplified by a CD-recordable (CD-R) drive device pursuant to the standard for the so-called compact disc.
The optical disc employed in this CD-R drive device is a so-called write-once type optical disc in which a strong laser beam is radiated on recording layer portions between pre-grooves on the optical disc, which are pre-formed guide grooves, for modifying the optical properties of the recording layer in order to permit the information to be recorded only once and for all on the disc.
Specifically, with the CD-R device, recording data is modulated with eight-to-fourteen modulation (EFM) for generating a modulated signal S1 in which the probability of occurrence of logical "1" and that of occurrence of logical "0" will be equal to each other, as shown in FIG. 1A. A laser beam is radiated from a laser diode based on the modulated signal S1 as reference. The laser beam is intermittently radiated on the optical disc in association with the logical level of the modulated signal S1. This generates regions of low reflectance, that is pits, in the recording layer portions between the pre-grooves. The laser diode is driven at this time at a high output.
The modulated signal S1 is generated so that a high level and a low level will continue within a time interval range of 3T to 11T, T being a reference period. This sequentially produces pits P for effecting data recording. The high reflectance regions in which the pits have not been formed are termed lands.
During data reproduction, the laser diode is driven at a low output for radiating a output laser beam on the optical disc. The laser light reflected from the optical disc irradiated with the laser light beam is received by a photodetector. The playback signal having its signal level changed responsive to the volume of the reflected light as shown in FIG. 1C, that is the RF signal, is produced. The signal level of the RF signal is detected with a slice level SL as a reference for detecting playback data shown in FIG. 1D.
Since the recording signal S1 is generated by EFM so that the probability of occurrence of logical "0" and that of logical "1" will be equal to each other, the slice level SL is selected so that the probability of occurrence of logical "0" and that of logical "1" will also be equal to each other with the playback data D1. This diminishes the bit error rate.
On the other hand, during data recording, the pit size is changed responsive to changes in the ambient temperature or in the laser wavelength, even although the laser diode is driven at a constant power for radiating the laser beam. For this reason, the laser diode driving power is sequentially switched in recording test data in a trial test region on the optical disc. The test data so recorded is reproduced for detecting an asymmetry value Asy for each step of the driving power. The asymmetry values can be detected easily using an asymmetry detection circuit. Of the detected asymmetry values Asy, that asymmetry value Asy closest to a pre-set asymmetry value Asy is selected. The driving power when the selected asymmetry value Asy is obtained is set as an optimum value of the driving power of the laser diode.
The asymmetry value is the time average ratio between pits and lands. Specifically, with respect to the reproduced RF signal from the optical disc, having a waveform shown in FIG.2, the asymmetry value is represented based upon the relation between the slice level SL for the playback data D1 shown in FIG. 1D which will give an equal probability of occurrence of the logical "0" to that of the occurrence of the logical "1" on one hand and the peak and bottom levels of the playback signal on the other hand. That is, the asymmetry value is given, using a peak level X.sub.1 and a bottom level X.sub.4 for the pulse width lit and a peak level X.sub.2 and a bottom level X.sub.3 for the pulse width 3T, by the following equation (1): ##EQU1##
With the above-described CD-R drive device, such asymmetry value of the recording signal recorded on the optical disc is selected which will give a minimum data error rate when decoding RF signals produced on reading out the recorded signal, such as by conversion into bi-level data. The asymmetry value which will give the minimum data error rate is governed by characteristics of the optical system of the CD-R drive device, laser light emitting time or optical disc characteristics. Thus the target asymmetry value differs with the machine versions or the manufacturers of the CD-R drive device.
For example, there may be an occasion wherein the target asymmetry value of a CD-R drive device manufactured by a company A is 0%, that of a CD-R device manufactured by another company B is -5%. If, in such case, data recording is made by the CD-R device manufactured by the company A up to a mid portion of an optical disc, and subsequent data recording is made by the CD-R drive device manufactured by the company B by way of supplementary data recording, the asymmetry value of the RF signal varies abruptly by 5% before and after the start of the supplementary data recording.
Although the slice level for converting the RF signal into a corresponding hi-level signal with the CD drive device or the CD-R drive device is configured for being changed during reproduction depending upon the asymmetry value of the RF signal, such change in the asymmetry value can be followed only within a range of tens of kilohertz.
Thus, when reading out data from an optical disc, on which supplementary data recording has been made on the CD drive device or the CD-R drive device in such as manner that the asymmetry value is changed abruptly ahead and at back of the start of the supplementary recording, the slice level cannot follow changes in the asymmetry value before and at back of the start of the supplementary data recording, thus frequently producing errors when converting the RF signal into a corresponding hi-level signal.